Method and device for measuring rotary speed

ABSTRACT

A method for measuring the rotary speed of a direct-current motor having a rotor and at least a shunt-wound field winding, wherein the field current includes an ac-component caused by mass-inhomogeneities in the rotor, including the steps of: frequency analyzing with a frequency analyzer the ac-component to form a frequency spectrum; selecting with a computing device a part of the frequency spectrum related only to the rotary speed; and determining from the selected part of the frequency spectrum the rotary speed of the motor.

CROSS-REFERENCE TO RELATED APPLICATION:

This application is a continuation-in-part of application Ser. No.330,829, filed Mar. 30, 1989, Pat. No. 5,042,080.

The invention relates to a method and device for measuring rotary speedand, particularly, for measuring the rotary speed of a direct-currentshunt-wound motor or of the rotating part of an electro-magneticallyactuated brake. It has been known heretofore to measure rotary speedwith analog and digital rotary-speed sensors. However, the added cost ofthe sensor and, in many applications, the expense for additionalconductors or leads is disadvantageous.

Further known heretofore is a method of measuring rotary speed from thecommutating ripple of the armature current (Elektronik Sonderheft"Electronics Special Supplement" 1986, No. 230, Page 55). A disadvantageof this heretofore known method, however, is that the number ofcommutation segments must be known.

In the case of a printing machine, speed regulation is effected in aconventional manner by a rotary speed sensor mounted on the machine. Inthis heretofore known method, the drive motor does not have any rotaryspeed sensor at its disposal. The rotary speed guidance is effected bythe regulation of the armature voltage. The drive of the printingmachine by the motor occurs via a belt transmission having atransmission ratio selected, respectively, to adapt or match thevariable maximum rotary speed of the printing machine to the fixedmaximum rotary speed of the motor. For an optimal regulation of therotary speed of the printing machine, however, not only the speed of themachine but also the speed of the motor must be determined, for whichpurpose another rotary speed sensor is used in the conventionalmachines.

It is accordingly an object of the invention to provide a method anddevice for measuring rotary speed of, for example, a direct-currentshunt-wound motor without using a rotary speed sensor therewith. Inparticular, it is an object of the invention to provide a method which,when applied to a machine, preferably a printing machine, serves toascertain the transmission ratio between the drive motor and a shaft ofthe machine driven by the motor via a transmission.

With the foregoing and other objects in view, there is provided inaccordance with one aspect of the invention, a method of measuring therotary speed of an electromagnetically actuated rotating device havingat least one winding carrying a current, which comprises analyzing thefrequency of a voltage corresponding to an alternating component of thecurrent.

In accordance with another measure of the method of the invention, therotating device is a direct-current shuntwound motor and the voltagefrequency which is analyzed corresponds to the alternating component ofthe field current of the motor.

In accordance with an alternate measure of the method of the invention,the device is a brake having a rotating part and the current carryingwinding, and the voltage frequency which is analyzed corresponds to analternating component of the current of the winding of the brake.

In accordance with a further measure, the method of the inventionincludes determining spectral components which are to be associated withrotary speeds lying within limits of a range of rotary speeds selectedon the basis of other operating conditions.

The method of the invention make it possible to effect economies bydispensing with the use of a rotary speed sensor. A further advantage isapparent in that the method of the invention is to a great extentindependent of the construction of the direct-current shunt-wound motor(number of poles, number of lamellae, and so forth) and of the brake,respectively. The method of the invention is suited for use in variousdifferent drive systems. The spectral components or lines produced inthe frequency analysis which are unsuited for measuring the rotaryspeed, such as for example the mains frequency and the harmonicsthereof, may be eliminated rather simply in many of these applications.

In accordance with an additional measure, the method also comprisesapplying a fast Fourier transformation (FFT) to the alternating voltageand passing off the frequency of a spectral line lying within the limitsof the speed range as the speed. This results in a special advantage dueto the use of a fast Fourier transformation (FFT) for which suitablesignal processors are available.

The task of determining the rotary speed is frequently undertaken inorder to ascertain when a motor reaches a prescribed speed. A solutionto this task is offered by providing an added measure in the method ofthe invention which includes feeding the alternating voltage to asteepsided band pass having an output to which a comparator isconnected, varying the rotary speed of the motor within a prescribedrange, and passing off the rotary speed corresponding to the limitingfrequency of the band pass as the rotary speed effective at the time avariation in an output voltage of the comparator occurs.

In accordance with yet another measure, the method includes checking atransmission ratio between the motor and a shaft of a machine connectedvia a transmission to the motor, and comprises measuring the rotaryspeed of the shaft, determining the prescribed rotary-speed range fromthe measured rotary speed of the shaft and a prescribed range for thetransmission ratio, and correlating the rotary speed of the motor andthe rotary speed of the shaft.

In accordance with another aspect of the invention, there is provided adevice for measuring the rotary speed of a direct-current shunt-woundmotor connected to the shaft of a machine via a transmission comprisinga rotary speed sensor attached to the shaft, means for controlling therotary speed of an armature of the motor means connected to a field coilof the motor for generating a field current therein, a current/voltagetransducer connected to the field current circuit and having an outputlinked via a frequency analyzing device to a microcomputer, and therotary-speed sensor having an output for feeding signals to themicrocomputer.

In accordance with another feature of the device of the invention, thetransmission is a belt transmission. By changing the belt pulleys, thetransmission ratio can be determined automatically and introduced intothe system for controlling the machine.

In accordance with a further feature of the device of the invention, thefrequency analyzing device is a signal processor equipped for performinga fast Fourier transformation (FFT).

In accordance with an additional feature of the device of the invention,the frequency analyzing device comprises a steepsided band pass, arectifier circuit and a comparator connected in series, the means forcontrolling the rotary speed of the motor armature being connected toand controllable by the microcomputer.

In accordance with yet another measure, the method of the inventionincludes selecting a spectral line suited for evaluation whenever apreviously selected spectral line is found to be unsuitable forevaluation.

In accordance with a concomitant feature of the method of the invention,the selected spectral line suited for evaluation is that of an harmonic.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a method and device for measuring rotary speed, it is neverthelessnot intended to be limited to the details shown, since variousmodifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings, in which:

FIG. 1 is a plot diagram of the spectrum of the alternating component ofa field current of a direct-current shunt-wound motor at a given rotaryspeed;

FIG. 2 is a diagrammatic and schematic view of a first embodiment of thedevice for measuring rotary speed according to the invention;

FIGS. 3a and 3b are time rate-of-change diagrams of signals generatedwith the embodiment of the device according to FIG. 2;

FIG. 4 is a view similar to that of FIG. 2 of a second embodiment of theinventive device;

FIG. 5 is a flow chart of a program for a microcomputer employed in thedevice according to the invention.

In the figures, identical parts are identified by the same referencecharacters.

Referring now to the drawing and first, particularly, to FIG. 1, thereis shown therein by way of example a spectrum of the field current of adirect-current shunt-wound motor rotating at a rotary speed of forexample 20.4 revolutions per second which equals 1,224 revolutions perminute. Accordingly, the spectrum contains a characteristic line L1 atthis frequency. Other lines L2 and L3 at frequencies of 50 Hz and 100 Hzresult from the residual ripple of the direct current gained from amains alternating current power supply. While the presence of lines at50 Hz and at 100 Hz is independent of the respective rotary speed, theother lines, especially the line L1, are shifted with the rotary speed.

The other lines that are shifted with the rotary speed are caused bychanges in the magnetic field between armature and stator stemming fromthe unavoidable irregularities in the motor armature.

These irregularities are caused for example by the winding slots in thearmature, which cause an a alternating wave form to be superimposed onthe field current. In case the number of winding slots is known, therotary speed of the motor can be determined from a Fourier-analysis ofthe alternating wave form. There are however also other irregularitiesin a motor armature, which are unintentional but are neverthelesssuitable for determining the rotary speed. These other irregularitiesstem from the unavoidable tolerances of the manufacturing process. Forexample, the rotor is never perfectly round, nor is it perfectlycentered on its shaft.

Also, the armature body may be attached to the shaft by a single key andkeyway which also creates an unsymmetry. Asymmetries caused by theseeffects operate to generate inhomogenities in the electrical field whichin turn lead to an alternating current component superimposed on thefield current. This alternating current can be dissolved onto itsvarious harmonies by means of Fourier analysis, and these harmonies canbe used to determine the rotary speed of the motor, as described in moredetail below.

In the embodiment of the device for measuring rotary speed according toFIG. 1, a direct-current shunt-wound motor is provided having anarmature 1 and a shunt field winding 2. The armature 1 is connected to acontrollable direct-current voltage source 3, while the field winding 2obtains direct-current voltage from the alternating-current mains via arectifier arrangement 4. A belt pulley 5 is mounted on a shaft of thedirect-current shunt-wound motor, the belt pulley 5, together with abelt 6 and a further belt pulley 7 forming a transmission by the aid ofwhich the direct-current shunt-wound motor drives a machine 8, which isrepresented only diagrammatically in the figure. Additionally located ona drive shaft 9 of the machine 8 is a rotary speed sensor 10.

The field current is measured with the aid of a current sensor ortransducer 11 which operates, for example, in accordance with the Hallprinciple and delivers a voltage proportional to the field current. Onlythe alternating component, as such, is required for performing themethod of measuring rotary speed according to the invention. Thisalternating component is in any case not allowed to pass through theband-pass filter 12. The band-pass filter 12 has at least one relativelysteep side of the bandwidth which is passed through the filter. Therelatively steep side of the band pass filter 12 is at the frequencywhich corresponds to a rotary speed of the motor at which the rotaryspeed of the machine is to be measured and, therewith, the transmissionratio is to be established.

A rectifier circuit 13 and a comparator 14 are connected to the outputof the band-pass filter 12. The output of the comparator 14 is linked toan input of the microcomputer 15 to which also the pulses generated bythe rotary speed sensor 10 are fed.

In the device according to FIG. 2, the transmission ratio which isdetermined from the sizes of the belt pulleys 5 and 7, can vary frommachine to machine because it is necessary to adapt or match thevariable maximum number of prints of the printing machine to the fixedmaximum rotary speed of the motor. Thus, for example, in a machine whichhas, in fact, been built, the print count i.e. the number of prints, is11,000 with a diameter of the belt pulley which is 142 mm whereas theprint count is 12,000 when the corresponding diameter is 155 mm.

The function of the device according to FIG. 2 is explained hereinafterwith regard to the diagrams shown in FIGS. 3a and 3b. The speed and thetransmission ratio, respectively, can be determined during a trial runprovided therefor, for example, after the machine has been started upfor the first time. It is also possible, however, during normaloperation if suitable rotary speeds exist during a suitable period oftime.

Controlled by the microcomputer 15, the armature 1 of the direct-currentshuntwound motor is supplied with such voltage that the rotary speedpasses through a prescribed range according to FIG. 3a. The rotary speedof the motor rises, in this case, from nmot-u to nmot-o, while the speedof the machine rises from nma-u to nma-o. The latter is determined withthe aid of the pulses generated by the rotary speed sensor 10. The input16 of the microcomputer 15 is active only during this rotary-speedrange.

The rotary-speed range is selected in such a way that on the one hand,it is possible to determine all of the transmission ratios which occurand, on the other hand, the determination of the spectral line is clearand unequivocal. If, for example, the reference voltage fed to thecomparator at 17 is so high that, in the spectrum according to FIG. 1,the lines L1, L2 and L3 are registered or detected, the rotary-speedrange of the motor can lie, for example, between nmot-u =16 revolutionsper second and nmot-u=24 revolutions per second. While this rotary-speedrange of the motor is being passed through, only the line L1 is in thecorresponding spectral range.

The band-pass filter 12 is then adjusted or tuned in such a way that itslower side lies at that frequency at which the transmission ratio is tobe determined and checked. This rotary speed nmot-actual is reached attime tm, which is represented in FIG. 3b with a signal jump j at theoutput 16 of the comparator 14.

At this instant, the value nma-actual of the machine's rotary speed isread into the microcomputer 15. By means of a division of the rotaryspeed of the motor, which at this time corresponds to the steep sidefrequency of the band-pass filter 12 and of the machine rotary speedread-in as a speed value from the speed sensor 10, a determination ofthe transmission ratio tr can be mode by the micro-computer 15 bydividing the read-in speed by the motor speed corresponding to the steepside frequency of bandpass filter 12. This value can be computed by themicrocomputer 15 and delivered via a data bus 15a, for example to asuperior control system for the machine. As the rotary speed increasesfurther, the upper limiting frequency of the band-pass filter 12 isexceeded, if necessary or desirable, so that the output voltage of therectifier circuit, as represented in FIG. 3b, falls again.

In the embodiment of the device according to FIG. 4, the spectralanalysis occurs with the aid of a signal processor 21 to which thealternating component of the field current is fed via a high pass filter22, a spectrum analyzer 19, and an analog/digital converter 20. In aconventional manner, the signal processor 21 is programmed to implementa fast Fourier transformation (FFT). Detailed information forprogramming a signal processor of the type TMS 320 of the company TexasInstruments for the FFT can be found in R. Kung, "Flexible FFT processorsolves real-time tasks", Elektronik ("Electronics") 21, Oct. 17, 1986,Pages 101 to 110. The result is fed to the microcomputer 15. In thisregard, it is not necessary to pass the signal through a prescribedrotary speed range as in the embodiment of the device according to FIG.2. The rotary speed ratio is determined only through the rotary speedindicated by the spectral line L1 and the output signal of the speedsensor 10 indicating the rotary speed of the machine 8 if the rotaryspeed is within the prescribed range. FIG. 5 shows a flow chart of aprogram for the microprocessor 15 with which a determination isperformed as to whether the transmission ratio tr-actual corresponds tothe given transmission ratio tr-nominal. For this purpose after startingup the appropriate program at 23, a nominal rotary-speed valuenma-nominal for the machine and a transmission ratio tr-nominal areprescribed (program step 24). Thereafter, at 25 the actual rotary speedof the machine nma-actual is read in and compared at 26 with the nominalrotary speed of the machine nma-nominal. If the rotary speeds do notagree, a loop consisting of the program steps 25 and 26 is repeated.Only if the rotary speeds agree does measuring of the frequency occur at27 for which the corresponding frequency of the spectral line L1(FIG. 1) is received in the microcomputer from the signal-processor 21(FIG. 4).

In program step 28, the transmission ratio tr-actual is then calculatedfrom the respectively determined and prescribed values fmot and n-ma.The transmission ratio tr-actual is compared at 29 with the nominalvalue for the transmission ratio tr-nominal. If the transmission ratiovalues are in agreement, the correctness of the transmission ratio isreported at 30, and if they do not agree, an error report is issued at31.

In some cases of application, especially in printing machines with onlyone or two printing units, the moment curve of the load over the angleof rotation is very irregular or nonuniform. If, moreover, aparticularly soft rotary speed regulator or control is installed, morepronounced fluctuations in the rotary speed of the drive occur. Thedetermination of the rotational speed of the motor is thereby renderedmore difficult. In accordance with the dynamics of the various differentrotary speeds, it is necessary to obtain an evaluation with sufficientrapidity.

If the working speed of the evaluation circuit i.e. of the microcomputer15, in the illustrated embodiments is too slow to follow thefluctuations of the rotary speed, multiple measurements should be takenand a subsequent mean value determination should be made.

The invention is not limited to the illustrated embodiments but can berealized in practice by a person of ordinary skill in the art also inother ways. Thus, for example, by using a suitable program in themicrocomputer, another spectral line can be sought and evaluated if thespectral line which was originally intended for evaluation is too weakor subject to disturbances. Further embodiments are possible by means ofother signal filtering methods and circuits.

I claim:
 1. A device for determining the transmission ratio of a machinedriven by a direct current motor by measuring the rotary speed of thedirect-current motor, the motor having a rotor and at least a shuntwinding drawing a field current having an ac-component created byinhomogeneities in the rotor, the device comprising a bandpass filterhaving a steep side of the pass band, said steep side being at afrequency corresponding to the rotary speed to be measured, a rectifierhaving an input connected to the bandpass filter forming a rectifiedsignal output; a comparator having one input connected to a referencevoltage, another input connected to the rectified signal output, and acomparator output being active when said comparator inputs are equal;said active comparator output indicating said rotary speed as beingequal to the frequency of said steep side of the bandpass filter.
 2. Adevice according to claim 1, including a Hall sensor coupled to saidfield current for sensing said ac-component, and an output of said Hallsensor coupled to said bandpass filter for transferring saidac-component.
 3. A device for measuring the rotary speed of adirect-current motor having a rotor and an least a shunt winding drawinga field current having an ac-component created by inhomogeneities in therotor, comprising spectrum-analyzing means coupled to the field currentfor forming a frequency spectrum of the ac-component; a Fourier analyzercoupled to the spectrum-analyzing means for forming a Fourier analyzedsignal of the frequency spectrum, and computing means coupled to theFourier analyzed signal for computing the rotary speed of the motor onthe basis of the Fourier analyzed signal, a Hall sensor coupled to saidfield current for sensing said ac-component; and an output of said Hallsensor coupled to said bandpass filter for transferring saidac-component; a machine rotating at a machine rotary speed, coupled tosaid motor; speed sensing means coupled to said machine having a sensedspeed output, and another input of said computing means coupled to saidsensed speed output for varying the rotary speed of said machine untilthe rotary speed of the machine is equal to the frequency of the steepside of the bandpass filter.
 4. A device according to claim 3, includinga machine rotating at a machine rotary speed, coupled to said motor;speed sensing means coupled to said machine generating a machine rotaryspeed signal, and another input of said computing means coupled to saidspeed sensing means for computing a transmission ratio between saidmotor and said machine as the ratio of the rotary speed of the motor andthe machine rotary speed at the time said comparator output is active.5. A device according to claim 3, including a transmission having agiven transmission ratio for coupling said motor to said machine,wherein said transmission is a belt transmission.